In facsimile, character recognition and other type scanning systems, the optical scanner accuracy is of considerable importance. The more accurate the scanner apparatus the higher the bit rate can be with increased efficency. Motor bearings, gear teeth, etc., must be accurately machined in order to ensure that the scanning apparatus is usable at the resolution desired. Further, slight weight discrepancies of the bearings, gear teeth, etc., in addition to the moving optical material, could also cause wobble or other vibration effects which detract from the efficient and accurate operation of the scan mechanism.
One prior art solution to this problem is seen in U.S. Pat. No. 4,002,830, issued Jan. 11, 1977. A reflecting mirror which is adjustable through all three axes via an electrical feedback system, corrects for facet to facet differences in a rotating polygonal mirror optical scanning system.
According to the present invention, a pentaprism monogon is rotated about an axis passing through its sides and through the axis of the applied light. Any wobble-inducing inaccuracies in the motor bearings, support, or drive mechanics are ineffective in causing any perturbation in the scanning light beam. The use of a pentaprism permits the scan to be virtually wobble free since rotation of a pentaprism about an axis passing through its sides (non-reflecting faces) does not cause beam reflectance angular changes. That is, the output light beams are in parallel and can be focussed by a postprism lens at the focal point of the lens which is made coincident with the scan plane. The present invention, therefore, permits a significantly simpler and more cost effective scanning technique.